Methods for use in detecting seismic waves in a borehole

ABSTRACT

The invention provides methods and apparatus for detecting seismic waves propagating through a subterranean formation surrounding a borehole. In a first embodiment, a sensor module uses the rotation of bogey wheels to extend and retract a sensor package for selective contact and magnetic coupling to casing lining the borehole. In a second embodiment, a sensor module is magnetically coupled to the casing wall during its travel and dragged therealong while maintaining contact therewith. In a third embodiment, a sensor module is interfaced with the borehole environment to detect seismic waves using coupling through liquid in the borehole. Two or more of the above embodiments may be combined within a single sensor array to provide a resulting seismic survey combining the optimum of the outputs of each embodiment into a single data set.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/431,872, filed May 7, 2003, entitled, METHODS AND APPARATUS FORDETECTING SEISMIC WAVES IN A BOREHOLE, which is incorporated herein byreference in its entirety.

GOVERNMENT RIGHTS

The United States Government has certain rights in this inventionpursuant to Contract No. DE-AC07-99ID13727, and Contract No.DE-AC07-05ID14517 between the United States Department of Energy andBattelle Energy Alliance, LLC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to seismic surveying ofsubterranean geological formations. More particularly, the presentinvention relates to improved seismic sensors for monitoring seismicwaves at a location within a liquid-filled borehole, and methods fortheir use.

2. State of the Art

Seismic surveying is used, by way of example, to examine subterraneangeological formations for the potential presence of reserves ofhydrocarbons such as petroleum, natural gas and combinations thereof aswell as the extent or volume of such reserves. Seismic waves, alsotermed acoustic waves, are emitted from a seismic energy source topenetrate through layers of rock and earth, and under certain conditionsare reflected and refracted by variations in the composition of thesubterranean formations in the path of the waves. Seismic sensorsconfigured as motion sensors in the form of geophones or accelerometersor pressure sensors in the form of hydrophones receive the reflected andrefracted waves and convert them into corresponding electrical signals,which are then analyzed for the presence and extent of formationscontaining oil and gas deposits.

An increasingly common technique for subterranean exploration is knownas borehole seismic surveying, wherein one or more seismic sensors areplaced below the earth's surface in the liquid-filled borehole of awell. The seismic energy source may be located above acquired thatprovides more detailed information about the surrounding area than maybe acquired using surface-based seismic sensors. These higher resolutionviews of subterranean formations can thus be examined for the presenceof hydrocarbon reserves that might otherwise remain hidden.

In order to reduce the time required for data acquisition, an array ofseismic sensor modules is deployed in the borehole to take simultaneousreadings at different locations along its length. The sensor modules,typically in the form of sondes containing geophones, are lowered intothe borehole on an elongated structure including a conductive cable suchas a wireline, tubing string or other suitable structure. The geophonesare configured for measuring the seismic waves in three directions oraxes, to give a reading for each of the orthogonal components of thewaves. For optimum sensing by the geophones, it is necessary that therebe a good interface between the sondes and the subterranean formationvolume surrounding the borehole to ensure effective transmission ofseismic energy. In the prior art this has often been accomplished byusing extendable mechanical arms that urge the sondes into firm contactwith the borehole wall, and provide an improved mechanical coupling forconducting waves to the geophones. In boreholes that are lined withmetallic casings, magnetic means have also been used in an attempt tocouple sondes to the borehole wall. All of the foregoing types ofsystems are controlled from above the surface to deploy the interfacestructures for the geophones, and involve complicated mechanisms forextending and retracting arms or orienting and activating magnets.Limitations on transmitting electric and hydraulic power to significantdepths are another significant concern. The prior art approaches resultin increased equipment cost and enhanced possibility of a malfunctioncausing the sondes to become stuck within the borehole and requiring anexpensive retrieval, or “fishing,” operation. Further, wave componentstraveling perpendicular to the borehole, versus wave componentstraveling up and down the borehole liquid column, are subject todifferent influences on their propagation. Interfacing all of the sondesin the same wall-coupling manner may not improve geophone readings forall three x, y and z sensor directions.

What is needed, therefore, are robust and uncomplicated methods andapparatus that achieve an improved interface between seismic waves andsensor modules within a borehole to provide high-resolution seismicsurvey data, while overcoming the problems associated with the priorart.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, improved seismic sensors andmethods for coupling them within a borehole are disclosed. Embodimentsof the present invention are directed to sensor modules having geophonesfor sensing of seismic waves. The sensor modules are lowered into aborehole as part of a sensor array, and monitor signals emitted from aseismic energy source located at the earth's surface or similarlycontained within a borehole. The sensor modules are interfaced with thesurrounding environment in such a way that complex and unreliablecoupling mechanisms are not required, while still enabling optimizedgeophone sensing.

In a first exemplary embodiment of the invention, a sensor moduleincludes a sensor package, or sonde, that is magnetically coupled to thewall of a borehole having a metallic casing. During deployment, thesensor module uses a self-contained device to automatically extend andretract the sensor package. Bogey wheels on the module ride along theborehole casing and operate a mechanism that retracts the sensor packageaway from the casing during sensor module travel. When the sensor moduleis brought to a halt at a desired sensing location, the bogey wheels nolonger operate the retraction mechanism, and the sensor package extendsfor magnetic coupling to the metallic casing. Upon renewed longitudinalmotion through the borehole, the bogey wheels and associated retractionmechanism detach the sensor package from its magnetic coupling. Thedevice thus provides a good interface with the subterranean environmentsurrounding the borehole, while eliminating complicated parts andlengthy connections to above surface actuation controls. In addition,because movement of the sensor module automatically retracts the sensorpackage, the risk of sticking the module within the borehole due to amalfunction is significantly reduced.

In a second exemplary embodiment of the invention, a sensor module isinterfaced by magnetic coupling to a metallic borehole casing asdescribed above with respect to the first exemplary embodiment, butwithout the requirement for any mechanical coupling devices. The sensormodule comprises a sonde having a plurality of permanent magnets placedaround its periphery. The magnets are attached so as to form protrusionsextending from the sides of the sonde. Each magnet is oriented such thatits protrusion presents a magnetic pole opposite to the pole presentedby the protrusion of an adjacent magnet. This creates magnetic fieldlines, which pass from one protrusion to another along the periphery ofthe sonde. The sensor module is simply dragged along the casing wall ofthe borehole during deployment, with some of the protrusions in magneticcontact with the borehole casing. This approach has the added advantageof scraping away surface deposits that may exist on the casing, whichwill improve the magnetic coupling.

In a third exemplary embodiment of the invention, a sensor module isdesigned to efficiently interface with the surrounding environmentwithout requiring direct coupling to a borehole wall or casing. Rather,the module is formed as a container or sonde having a mass-to-volumeratio that gives it an average density substantially equal to that ofthe borehole liquid. This equal density, and the nearly incompressiblenature of a liquid, allows the sensor module to precisely match thedisplacement of borehole liquid due to seismic wave disturbance. Thiscreates, in effect, a liquid coupling wherein the motion of the sensormodule can be monitored to exactly track the seismic waves. The simpleand lightweight construction of this embodiment is highly cost effectiveand reduces the need for complicated supporting architecture,facilitating its deployment on wireline. This type of sensor module isalso well adapted for attachment to drill pipe or coiled tubing used toperform borehole drilling or downhole maintenance and remediationfunctions, and may be particularly suitable for use in seismic whiledrilling operations.

In yet another exemplary embodiment of the present invention, a sensorarray having a number of sensor modules of the various above-describedembodiments is provided for deployment within a borehole. Geophoneswithin the sensor modules measure seismic waves emitted from a seismicenergy source, and provide an output reading for each of the orthogonalcomponents of the waves. The wave components most effectively measuredby each of the sensor module embodiments are then used to generate aseismic survey, while the other components are filtered out. Thisoptimizes the survey data by combining the advantages of each sensingtechnique into a single result.

Other and further features and advantages will be apparent from thefollowing detailed description of the invention taken in conjunctionwith the accompanying drawings. The following examples are provided forpurposes of illustration only, and are not intended to be limiting. Itwill be understood by one of ordinary skill in the art that numerouscombinations and modifications are possible for the embodimentspresented herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIG. 1 is a side view of a well borehole having a sensor array deployedtherein in accordance with the present invention;

FIG. 2 is a perspective view of a well borehole indicating the x, y andz directions for orthogonal components of seismic waves passing throughthe borehole and surrounding subterranean area;

FIGS. 3A and 3B show schematic views of a magnetic sensor module of thepresent invention having a self-contained device to automatically extendand retract a sensor package;

FIGS. 4A and 4B show schematic views of one alternative mechanism forretracting the sensor package of the magnetic sensor module depicted inFIGS. 3A and 3B;

FIG. 5 shows a schematic view of another alternative mechanism forretracting the sensor package of the magnetic sensor module depicted inFIGS. 3A and 3B;

FIG. 6A is a schematic side view of a magnetic sensor module of thepresent invention comprising a sonde with permanent magnets formingprotrusions around its periphery;

FIG. 6B is a schematic top view of the magnetic sensor module of FIG. 6Ataken along section line B;

FIG. 7 is a schematic view of an alternative attachment for the sensormodule depicted in FIGS. 6A and 6B;

FIG. 8 is a schematic view of a sensor module of the present inventionformed as a sonde that does not require direct coupling to a boreholewall or casing; and

FIGS. 9A and 9B show the sensor module of FIG. 8 including additionalphysical coupling mechanisms for vertical sensing.

FIGS. 10A and 10B show a sensor module that does not require directcoupling to a borehole wall or casing and that is configured forattachment to a drill pipe or coiled tubing.

FIG. 11 shows alternative embodiments to the sensor module depicted inFIGS. 10A and 10B.

DETAILED DESCRIPTION OF THE INVENTION

Although the subsequent examples will be discussed in terms ofdeployment in a well used for petroleum or gas exploration andproduction, it should be understood that the present invention wouldalso work well for seismic surveying applications not related to thesefields. Any technology that uses sensors deployed within a borehole tomonitor seismic waves may benefit from the present invention.

Referring initially to FIG. 1, three different sensor module embodiments14, 52 and 66 of the invention are illustrated in a sensor array 2.Sensor array 2 is deployed in a liquid or slurry filled well borehole 4for seismic surveying of the subterranean formation volume proximate thewell. Sensor array 2 is lowered into the borehole 4 on an elongatedstructure depicted as wireline 6, however, other suitable structure suchas a tubing string may be used. The liquid or slurry may comprise, forexample, water or a water and hydrocarbon based drilling fluid, or“mud.” In the case of petroleum exploration, the interior of borehole 4will usually be surrounded by a metallic, typically steel, casing 8which has been floated into the borehole 4 subsequent to the drillingthereof and then cemented in place, as known to those of ordinary skillin the art. Seismic waves 10 generated by seismic energy source 12 arepassed through the subterranean formations surrounding borehole 4, andsensor array 2 monitors these waves from within borehole 4 afterreflection from and refraction by these formations to provide geologicalinformation.

Turning to FIG. 2, the seismic waves may be considered as beingcomprised of orthogonal components traveling in the x, y and zdirections. The sensor modules of array 2 carry geophone type sensorsthat are configured and oriented for measuring the seismic waves inthese three directions, or axes, to give a reading for each of theorthogonal components of the waves. The geophones operate by measuringdisplacement between a stationary first part and a second part that isallowed to move along a defined axis in response to the seismic wavestransmitted thereto as vibrations. This method of operation requires agood interface between the geophones and the final transmission mediumfor the seismic waves acting upon the geophones in order to effectivelyreceive the vibrations.

FIGS. 3A and 3B schematically illustrate a first sensor moduleembodiment 14 of the present invention having a self-contained device toautomatically extend and retract a sensor package. Sensor module 14includes a housing 16, bogey wheels 18 and sensor package or sonde 20.At least one of the bogey wheels 18 is biased to swing away from housing16 so as to force the bogey wheels 18 on both sides of housing 16 intofirm contact with borehole casing 8. FIG. 3A shows that when the sensormodule 14 is stationary within borehole 4, sensor package 20 extends outfor magnetic coupling to borehole casing 8 via one or more permanentmagnets M carried by the sensor package. When sensor module 14 is placedin motion, for example during initial deployment or repositioning withinborehole 4, bogey wheels 18 are rotated due to their contact withborehole casing 8. The rotation operates a mechanism attached to bogeywheels 18 that retracts sensor package 20 away from borehole casing 8,as depicted in FIG. 3B. Sensor package 20 is held in the retractedposition during travel of sensor module 14. After movement has ceased,bogey wheels 18 no longer rotate to operate the retracting mechanism,and sensor package returns to the extended position for magneticcoupling to borehole casing 8.

FIG. 4A shows one implementation of the first exemplary embodiment,characterized as sensor module 14′. Sensor module 14′ employs ahydraulic pump and cylinder arrangement as the mechanism for retractingsensor package 120 carrying magnets M and one or more geophones G. Bogeywheel 118 a is biased by spring 22 to swing outwardly toward the wall ofcasing 8 in the direction of arrow 24 and press against casing 8. Bogeywheels 118 b and 118 c are pressed against the side of borehole casing 8opposite bogey wheel 118 a. A hydraulic pump P for driving pistonswithin a cylinder 26 through control manifold C is incorporated withbogey wheel 118 a. The hydraulic pump P, for example a gear pump, isactuated by rotation of the wheels. Other types of pumps using drivesconfigured for translating the wheel motion into pumping force wouldalso be suitable. In operation, hydraulic pump P pressurizes a hydraulicfluid responsive to rotation of bogey wheel 118 a as sensor module 14′moves longitudinally within casing 8, the hydraulic fluid pressure beingcommunicated to the center of cylinder 26 to move a piston therein (notshown, see FIG. 4B) outwardly in cylinder 26 to tension cable 32 andretract sensor package 120 against an outward spring bias provided bylinkages 36.

In FIG. 4B, which schematically represents the hydraulic pump andcylinder retracting mechanism, upon movement of sensor module 14′, thebogey wheel rotation actuates the hydraulic pump P, which in turnprovides pressurized hydraulic fluid to cylinder 26, extending piston 28in the direction of arrow 30. A cable 32 attached at one end 32 a to ram28 is routed along guide pins or pulleys 34 and attaches at its otherend 32 b to sensor package 120. As piston 28 extends in the direction ofarrow 30, it pulls cable 32, which in turn pulls on sensor package 120and draws it into a retracted position. During sensor module 14′ travel,cylinder 26 will be maintained in a retracted state with piston 28extended. Once motion is stopped, the hydraulic pump P will no longerpressurize cylinder 26, and sensor package 120, which is biasedoutwardly by linkages 36 represented schematically as springs in FIG.4B, will return to the extended position for magnetic coupling bymagnets M with the casing 8. Hydraulic fluid is permitted to bleed backinto a reservoir in control manifold C for reuse through a check valvewhich opens when hydraulic pump P is no longer pressurizing thehydraulic fluid.

It is possible that the retracting mechanism could use hydraulicallyactuated devices other than cylinder 26, such as by extending a bellowsor diaphragm with hydraulic pump P to pull cable 32. It is alsocontemplated that hydraulic pump P may be used to power a drive to winda cable to retract the sensor package 120 without using a hydrauliccylinder, using a slip clutch to prevent damage to the cable when thesensor package is fully retracted. With any of the above describedhydraulically actuated devices, the retracting mechanism should includea balanced hydraulic system capable of compensating for high pressuresthat may be encountered within the operating environment in order toallow the hydraulic fluid to bleed back into the reservoir in controlmanifold C. Such balanced hydraulic systems typically involve mechanismssuch as bellows or diaphragms for equalizing the pressure between thehydraulic fluid reservoir and the borehole fluid, as previously known inthe art.

Returning to FIG. 4A, to aid in overcoming the magnetic attraction ofmagnets M of sensor package 120 when initially detaching it fromborehole casing 8, sensor package 120 is supported in housing 116 withlinkages 36. As sensor module 14′ begins to move, linkages 36 permitrocking or twisting of sensor package 120 in the direction of arrow 38about an axis extending perpendicular to the plane of the drawing sheetto break the magnetic coupling with casing 8. This feature reduces theforce that would be required for detachment if sensor package 120 weresimply pulled back from the face of casing 8 in a directionperpendicular thereto, and thus assists retraction of sensor package120.

It is also desirable that sensor module 14′ include an automaticlatching mechanism to maintain the sensor package 120 in a retractedposition whenever the hydrostatic pressure within the borehole is low,i.e., near the surface. The latching mechanism could be mechanical,hydraulic, electronic or comprise any other form generally known in theart form providing such a function.

FIG. 5 shows another implementation of the first exemplary embodiment,characterized as sensor module 14″. Sensor module 14″ uses a slip orfriction clutch and cam bar or member arrangement as the mechanism forretracting a sensor package 220. Bogey wheel 218 a is mounted on springbiased arm 40 to swing out in the direction of arrow 42 and pressagainst casing 8. Bogey wheels 218 b and 218 c are pressed against theopposite side of casing 8 due to the spring bias of arm 40 on theopposing side of sensor module 14″. Sensor package 220 is mounted to acam bar 44. Cam bar 44 is eccentrically interfaced with bogey wheels 218b and 218 c via respective slip or friction clutches 46 a and 46 b. Whensensor module 14″ is stationary within borehole 4, sensor package 220 isin the extended position for magnetic coupling to casing 8, as depictedby FIG. 5. Upon longitudinal movement of sensor module 14″, the rotationof bogey wheels 218 b and 218 c in either direction will force cam bar44 in the direction of arrow 48, and retract sensor package 220. At acertain point, the force required for further displacement of cam bar 44in direction 48 will be sufficient to cause friction clutches 46 a and46 b to engage. Thereafter, cam bar 44 will be maintained at a constantretracted position during longitudinal travel of sensor module 14″through casing 8. When sensor module 14″ is repositioned to a desiredlocation, the longitudinal direction of travel of sensor module 14″ isreversed for a short distance, which may be a matter of inches,sufficient to release friction clutches 46 a and 46 b and causing cambar 44 to move back out and extend sensor package 220 for magneticcoupling by magnets M with casing 8.

Sensor package 220 is supported on cam bar 44 by a pair of staggeredpins 50. These act to aid in overcoming the magnetic attraction ofsensor package 220 when initially detaching it from casing 8 in much thesame way as linkage assembly 36 of sensor module 14′. As sensor module14″ begins to move, one of the pins 50 pulls on extended sensor package220 before the other, causing one side of sensor package 220 to liftfrom magnetic coupling with casing 8 before the other side to assistwith breaking the magnetic coupling to casing 8.

Turning to FIGS. 6A and 6B, a second exemplary sensor module embodiment52 of the present invention is illustrated. Sensor module 52 comprises asonde 54 containing sensors (not shown) and having a plurality ofpermanent magnets 56 placed around its periphery. The poles of magnets56 terminate in protrusions 58 that extend outwardly from sonde 54. FIG.6B, taken along section line B in FIG. 6A, shows permanent magnets 56 ofU-shaped cross-section with one pole at each tip. Other magnet shapesare within the scope of the invention, as long as they present aprotrusion terminated by a magnetic pole. Each magnet 56 is orientedsuch that each protrusion 58 has a magnetic pole oppositely charged fromthe pole of the protrusion adjacent to it. This creates magnetic fieldlines 60 passing from one protrusion 58 to another along the peripheryof sonde 54 and through borehole casing 8. During deployment orrepositioning within borehole 4, sensor module 52 is simply draggedalong borehole casing 8, with protrusions 58 in magnetic contacttherewith.

Coupling of sensor module 52 using permanent magnets 56 provides aninterface with the surroundings that does not require external power orcontrols and is devoid of moving parts. The simple design frees up spaceand conductive elements on wireline 6 for data transmission, allowingmore sensor modules or other equipment to be added to array 2. Movementof sensor module 52 with protrusions 58 in contact with borehole casing8 also has the added advantage of scraping away possible surfacedeposits, which will improve magnetic coupling.

Sensor module 52 may be connected to wireline 6 at a central attachmentpoint 62 a such that it is symmetrically balanced, as depicted in FIGS.6A and 6B. Under this arrangement, sensor module 52 will presentattachment surfaces around its perimeter that are uniformly disposed toattachment with borehole casing 8. As seen in FIG. 7, in some instancesit may be desirable for sensor module 52 to favor one side forattachment to borehole casing 8. Sensor module 52 is hung from wireline6 at an off-center attachment point 62 b, which will bias it to one side64. Thus, side 64 will have a predisposition for contact with boreholecasing 8. Under this arrangement, magnets 56 may also be limited to thecontact area of side 64, rather than being placed around the entireperiphery of sonde 54 to further ensure that magnetic coupling to casing8 will take place on side 64.

FIG. 8 schematically illustrates a third exemplary sensor moduleembodiment 66 of the present invention. Sensor module 66 is fabricatedto interface with the surrounding environment within borehole 4 withoutrequiring direct physical coupling to the borehole wall or casing 8.Rather, sensor module 66 uses a liquid type coupling wherein seismicwaves 10 are transmitted to the module via the borehole liquid 104.Sensor module 66 comprises a container or sonde 68 for carrying one ormore geophone type sensors 70. Sensors 70 as depicted in FIG. 8 areoriented to detect and measure the magnitude of seismic waves in the xand y directions, perpendicular to the longitudinal axis of borehole 4and, ideally, horizontal in orientation. Sensor module 66 is constructedso that sonde 68 and any sensors 70 contained therein have a combinedmass-to-volume ratio with an average density effectively equal to thatof the borehole liquid 104. In other words, sensor module 66 will beneutrally buoyant within the borehole liquid. Sonde 68 may comprise, byway of example, a low density solid structure surrounding sensors 70 ormay enclose a hollow volume within which sensors 70 are mounted. Sonde68 therefore presents a surface area that is accelerated at a rate equalto the displacement of borehole liquid 104 responsive to seismic waves10 transmitted thereto by casing 8 Further, the nearly incompressiblenature of a liquid means this displacement will transmit the seismicenergy directly to sensor module 66 without any variation in wavepropagation.

The response is directional and unmitigated for any frequency ofconcern. While the borehole liquid 104 is not capable of transmitting ashear wave, the result of solid shear disturbance (in the formation) isan orthogonal compressive wave which may, in turn, be detected. Thegeophones 70 of sensor module 66 are thereby effectively interfaced withthe surrounding environment by “coupling” to borehole 4 via the boreholeliquid 104. This eliminates the need for any mechanical coupling devicesand provides a highly economical and lightweight unit that is easilysupported within the borehole environment. For example, each sensormodule 66 may be fabricated for as little as ten percent of the cost ofa clamping type sensor module, and the cost of supporting equipment maysimilarly be significantly reduced. Significantly more potential usersexist due to the less extensive equipment requirements of thisembodiment, and operational time may be significantly reduced incomparison to clamping type sensor modules as well. Further, it isnotable that this embodiment of the invention is operable in an uncasedborehole, since there is no need for affixation of the sensor module tocasing for seismic coupling.

The fluid coupled type sensor module described above works best fortranslating the x and y, or horizontal orthogonal, seismic wavecomponents to corresponding geophone sensors 70 contained therein, asthe impedance mismatch between the solid (formation, cement, casing,etc.) and borehole liquid 104 is small, as is the length of seismic wavetravel through the borehole liquid 104. This is due to the fact thatwhile the borehole liquid is nearly incompressible, the z, or verticalseismic wave component, along the longitudinal axis of borehole 4 willtravel a much greater distance through the borehole 104 liquid unlessthe sensor is deployed at the bottom of the borehole 4, and any amountof liquid compressibility will have a cumulative effect. One way tocompensate for this problem is to incorporate sensor module 66 into anassembly having an exterior vertical geophone component 72 that isphysically coupled to the side of casing 8. FIGS. 9A and 9B show such anassembly wherein a housing 74 with a plurality (for example, four) bowsprings 76 circumferentially disposed thereabout holding vertical(z-axis) geophone sensors 72 mounted thereon in physical contact withthe side of borehole 4 through magnetic coupling using one or moremagnets M. Sensor module 66 is connected to housing 74 so as not tohamper its ability to be displaced by the borehole fluid. FIG. 9A, forexample, shows a sensor module 66 suspended below housing 74, while FIG.9B shows a unitary housing 74′ that allows free movement of sensormodule 66 suspended within its confines (only one bow spring 76 of aplurality shown for convenience). It is to be understood that otherphysical coupling means may be used for a Z-axis geophone sensor, theonly requirement being that they allow sensor module 66 to beaccelerated by the borehole fluid for liquid coupling. It is furthercontemplated that the bow spring type embodiments of FIGS. 9A and 9B maybe used to support and magnetically couple x, y and z-axis geophones tocasing, and such a configuration is within the scope of the invention.

While sensor module 66 has been depicted as being deployed on wireline 6as part of a sensor array, other downhole assemblies may also benefitfrom the use of fluid coupled type sensors. A borehole, for example, istypically drilled by using a bit that is suspended on a drill stringcomprising coupled sections of drill pipe extending downwardly into theborehole from the surface. Rotating the drill string at the surfaceusing a rotary table or top drive rotates the bit for drilling whenweight is applied to it through the drill string. The drill string mayinclude a bottom hole assembly above the bit including, for example, adownhole motor with a bent housing or other steering element or assemblyto enable guided, deviated or directional drilling of the borehole.Further, after an oil or gas well has been successfully drilled andcompleted, it is necessary over the productive lifetime of the well toperform maintenance or remediation operations within the well borehole.This maintenance or remediation often includes de-scaling operations, orreworking operations such as fracturing or acidization to increaseproduction in older wells. It is quite advantageous to be able to insertequipment into a borehole necessary to perform such maintenance orremediation without removing the surface production equipment at thewell head. Coiled tubing, which can be inserted into the boreholethrough the surface production equipment without removal thereof, hasbeen employed to carry out this function. More recently, coiled tubinghas also been used in conjunction with downhole motors for drillingoperations as well as other types of borehole operations.

When drilling, it is desirable to know what strata will be drilledthrough at any time in order to provide appropriate drilling parametersduring operation. Features of the strata ahead of the drill may therebybe anticipated, enabling optimized navigation of the borehole throughsubterranean formations which otherwise might damage the bit or exposethe well to dangerous gas overpressure regions. It would, of course, bepossible to extract the entire drilling assembly from the borehole andsend down a wireline-carried sensor array for surveying, but the timeand cost associated with such an approach are very high and safetyconcerns render this an undesirable alternative. In order to overcomethis problem, it is known in the prior art to include seismic sensorarrangements directly within a drilling assembly to examine the areadirectly surrounding the drill bit concurrently with drilling. Anexample of this method, often referred to broadly as “measurement whiledrilling” (MWD) although more accurately termed “seismic whiledrilling,” is disclosed in U.S. Pat. No. 5,798,488 to Beresford et. al.,which is incorporated herein by reference. Because rotation of the bitmust typically be stopped and circulation of drilling fluid ceased inorder to allow seismic measurements without interference from drillingvibrations and fluid turbulence, the fluid coupled sensors of thepresent invention would be well suited to such an MWD application. Byeliminating the need for any mechanical coupling devices, fluid coupledsensors according to the present invention may be activated with minimalpauses in drilling and circulation and may be more easily incorporatedinto a drilling assembly.

FIGS. 10A and 10B show an embodiment of a fluid coupled type sensormodule 166 that is configured for attachment to a drill pipe or coiledtubing 77, which have much greater diameters than a wireline. Whilesensor module 166 is depicted as disposed within casing 8, sensor module166 has equal utility for deployment within an uncased borehole for usein conducting seismic operations while drilling. As seen in side viewFIG. 10A, sensor module 166 comprises an annular housing 168 carryingone or more geophone type sensors 170. FIG. 10B shows annular housing168 surrounding drill pipe or coiled tubing 77 and attached thereto withhighly resilient mounts 172, allowing housing 168 to move freely in thex and y orthogonal directions. Mounts 172 may be formed, for instance,of low modulus rubber, springs or any other material having sufficientelasticity to allow housing 168 to move in the x and y directionswithout substantial resistance. Furthermore, while FIG. 10B shows fourmounts 172 for supporting housing 168, any number of mounts could beused, or the mounting structure could even be formed as a unitary ringentirely surrounding drill pipe or coiled tubing 77. Such an approachmay facilitate damping of seismic waves in the z direction along theaxis of the borehole. As with sensor module 66, sensor module 166 andsensors 170 contained therein have a combined mass-to-volume ratio withan average density effectively equal to that of the borehole liquid 104so that sensor module 166 and sensors 170 are essentially neutrallybuoyant. The geophones 170 of sensor module 166 are thereby interfacedwith the surrounding environment by the movement of annular housing 168.

If sensor module 166 is deployed on drill pipe 77 in a drillingoperation it may be desirable to employ concentric stabilizers 400 (FIG.10A) intermittently along drill pipe 77 to prevent contact of sensormodule 166 with the wall of the borehole. Centralization of thestructure (drill pipe or coiled tubing) carrying sensor module 166should always be considered if the borehole segment in which sensormodule 166 is deployed is off-vertical by any significant amount. Ofcourse, sensor module 166 may be placed along a necked-down or reduceddiameter central portion of drill pipe 77 between the diametricallyenlarged male (pin) and female (box) ends, which function to centralizethe drill pipe and maintain sensor module 166 out of contact with theborehole.

FIG. 11 shows a sectional side view of an alternative to the above fluidcoupled structure, wherein a sensor module 266 is fixedly mounted todrill pipe or coiled tubing 77. In this embodiment, geophone sensors 270are not interfaced with the environment by movement of housing 268, butare instead movably mounted within recesses 274 by resilient mounts 272for direct interface with borehole liquid 104. As seismic waves passthrough borehole liquid 104 in the x and y orthogonal directions,sensors 270 move within recesses 274 to monitor their transmission.Housing 268 may comprise an annular housing with recesses in itssurface, or may simply comprise shielding structures extending fromdrill pipe 77 to at least partially surround sensors 270. It is alsocontemplated that housing 268 may be completely omitted, and sensors270′ would be movably mounted on resilient mounts 272′ in recesses 274′formed directly in a specially configured drill pipe 77. While thesefluid coupled embodiments are depicted and described as including aannular ring type housing, it will be understood by those of ordinaryskill in the art that other housing configurations will be possible, andthat any number of geophone sensors may be positioned in variouslocations about and along drill pipe or coiled tubing 77.

When sensors 170, 270 are deployed on drill pipe or coiled tubing 77 ina borehole that has been drilled in a direction that is notsubstantially vertical, sensors 170, 270 will offset from the x and yorthogonal axis. It is necessary to mathematically compensate for thisoffset, which compensations are within the ability of those of ordinaryskill in the art and so will not be described in further detail herein.

It is also contemplated that an array of sensors 370 may be deployed ona conductive cable completely within coiled tubing 77 as shown in FIG.11 as yet another implementation of the present invention. In such aninstance, the material of coiled tubing 77 would be selected to “give,”or respond to, an encounter with a seismic signal to convey the same tosensors 370 disposed in a surrounding fluid 374 within coiled tubing 77for substantially neutral buoyancy and effective signal transmission.

In a further embodiment of the present invention, a number of sensormodules of the various different above-described embodiments areprovided for deployment within a borehole on a single sensor array.Going back to FIG. 1, sensor array 2 includes sensor modules 14, 52 and66 instead of only one module type or embodiment, as would be the casein prior art arrays. The sensor signals for the separate x, y and zorthogonal seismic wave components from each sensor module are output toa processor 78. The signals for wave components most effectivelymeasured by each of the sensor module embodiments are then used togenerate a seismic survey, while the other component signals arefiltered out. For example, the vertical wave component signal from theliquid coupled module 66 might be filtered out, while one or morehorizontal components of modules 14 and 52 are eliminated, depending onsignal strength and correlation between the sensor outputs. Thisapproach optimizes the integrity of survey data by combining theadvantages of each sensing technique into a single, composite output.The sensor module composition of array 2 in FIG. 1 is only for purposesof illustration and not by way of any limitation of the presentinvention. Any number of modules in any order on wireline 6 may be used.Moreover, only two sensor module embodiments might be deployed, insteadof the three shown in FIG. 1. It is further noted that all of theembodiments of the present invention, due to their simplicity, mayenable the use of arrays of dozens or even hundreds of sensor modulesdue to their light weight and simplicity of operation, as wirelinetransmission capacity may be used for data rather than power and controlfunctions.

It is also contemplated that the sensor modules of the present inventionmay be fabricated in multiple segments, wherein the geophone sensors andassociated signal amplification/transmission components are housedseparately. This approach reduces the sensor module mass for thegeophone containing segment and thus increases the effectiveness of themagnetic coupling force securing the geophone to the casing wall or, inthe case of the liquid coupling embodiment, the sensor moduledisplacement response. Although the present invention has been describedwith respect to the illustrated embodiments, various additions,deletions and modifications are contemplated without departing from itsscope or essential characteristics. Furthermore, while described in thecontext of oil and gas exploration, the invention has utility in alltypes of subterranean geological exploration. The scope of the inventionis, therefore, indicated by the appended claims rather than theforegoing description. All changes that come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

1. A method of coupling a seismic sensor within a borehole comprising:positioning a sensor module having a plurality of bogey wheels and asensor package within a liquid-filled borehole, wherein said pluralityof bogey wheels and said sensor package are in contact with a wall ofsaid borehole; moving said sensor module along said wall to rotate saidplurality of bogey wheels; and retracting said sensor package away fromsaid wall due to the rotation of said bogey wheels.
 2. The method ofclaim 1, further comprising: biasing at least one of the bogey wheels toswing out from the sensor module and press against the wall of theborehole.
 3. The method of claim 1, wherein retracting the sensorpackage comprises: pumping a fluid using the rotation of the bogeywheels; filling a hydraulically actuated device with the fluid to extenda portion of the hydraulically actuated device; and applying a pullingforce to the sensor package with the portion of the hydraulicallyactuated device.
 4. The method of claim 3, wherein the pulling force isapplied to the sensor package with a cable connected from the ram to thesensor package.
 5. The method of claim 3, wherein pumping a fluidcomprises: actuating a hydraulic gear pump contained within one of thebogey wheels.
 6. The method of claim 1, wherein retracting the sensorpackage comprises: supporting the sensor package on a cam bar;eccentrically interfacing the cam bar with at least one of the bogeywheels; rotating the bogey wheels to force the cam bar away from thewall of the borehole; and engaging a friction clutch connected betweenthe at least one of the bogey wheels and the cam bar to hold the cam barin a retracted position during movement of the sensor module.
 7. Themethod of claim 6, further comprising: extending the sensor package backtowards the wall of the borehole by reversing a direction of rotation ofthe bogey wheels.
 8. The method of claim 1, wherein the wall of theborehole comprises a metallic casing, and the contact of the sensorpackage with the wall includes magnetic coupling.
 9. The method of claim8, further comprising: lifting a first side of the sensor package priorto lifting a second side of the sensor package to break the magneticattachment and instigate the retracting of the sensor package away fromthe wall of the borehole.